The present invention relates to purification of polycarboxylic anhydrides such as phthalic anhydride and maleic anhydride, and especially maleic anhydride. More particularly, the present invention relates to obtaining anhydride products, especially a maleic anhydride product, of good color stability.
Polycarboxylic anhydrides such as phthalic anhydride and maleic anhydride are commercially produced by oxidation of a hydrocarbon feedstock using a catalyst to obtain specificity for the reaction. Typical feedstocks for phthalic anhydride production include naphthalene and orthoxylene. Maleic anhydride can be produced from a number of different hydrocarbon feedstocks, including benzene, butane, butene, furfural, and crotonaldehyde. Catalysts for both processes usually contain the oxides of vanadium, along with the oxides of other elements as modifiers.
Production of phthalic anhydride or maleic anhydride includes means to recover the anhydride from the oxidation reactor effluent, for example by absorption of the anhydride followed by stripping the anhydride from the absorbent to obtain a crude anhydride stream, and means to purify the recovered crude anhydride, for example by distillation to obtain product anhydride. One of the important specifications for the purified product anhydride is color stability. The color of phthalic anhydride or maleic anhydride is commonly measured by so-called APHA color standards using standard platinum-cobalt solutions prepared in accordance with ASTM D-2280-66 procedure. This color scale was developed by the American Public Health Association and is also known as the Hazen Platinum Cobalt Scale. A description may be found at page 2048 of the 5th edition of "Standard Methods of Chemical Analysis", by Wilford W. Scott.
Typical specifications on product maleic anhydride call for an APHA or Hazen color of 20 or lower for molten anhydride and a color of 40 or less after 2 hours heating at 140.degree.C. Good commercial maleic anhydride has a Hazen color below 125 after 24 hours at 140.degree.C. Because the color of maleic anhydride is particularly prone to degrade (become darker) upon heating and because color tests frequently call for measurement after a prolonged period of heating, the color stability of maleic anhydride is also frequently referred to as color thermal stability.
Various methods have been employed to obtain colorstable carboxylic acid anhydrides. For example, U.S. Pat. No. 2,129,166 discloses color-stability improvement by heating crude maleic anhydride for a considerable length of time and then distilling to obtain a product anhydride; U.S. Pat. No. 2,134,531 discloses the use of sulfuric acid to treat crude maleic anhydride followed by distillation to obtain a product maleic anhydride; U.S. Pat. No. 2,150,331 discloses the formation of a maleic anhydride compound with anthracene followed by separation of the anthracenemaleic anhydride compound and then decomposing the compound to obtain pure maleic anhydride vapor; U.S. Pat. No. 2,296,218 discloses heating crude maleic anhydride with a metal compound selected from the oxides and hydroxides of sodium, potassium, lithium, calcium, zinc and magnesium and the halides of zinc, iron and aluminum and then distilling the treated maleic anhydride to recover a product maleic anhydride; U.S. Pat. No. 2,308,588 discloses heating crude maleic anhydride with an oxide of boron and then distilling to obtain purified maleic anhydride; U.S. Pat. No. 2,831,896 teaches that alkali metal cations cause undesirable decarboxylation of maleic anhydride, especially above 150.degree.C, and that therefore if an alkali metal cation is inadvertently present a boron compound should be added before the crude maleic anhydride is distilled; U.S. Pat. No. 2,959,600 discloses adding paraldehyde to crude maleic anhydride, heating and then distilling to obtain purified maleic anhydride. British Pat. No. 1,204,846 and U.S. Pat. No. 3,564,022 disclose a process comprising
"treating crude maleic anhydride at a temperature of from about 130.degree.C. up to but not exceeding the boiling point of the maleic anhydride with a small amount of a heat stable acidic compound or mixture thereof, such as, the inorganic acids such as ortho phosphoric, meta phosphoric, pyrophosphoric, phosphorus, the oxides of phosphorus and sulfur such as P.sub.2 O.sub.3, P.sub.2 O.sub.4, P.sub.2 O.sub.5 and SO.sub.3 ; the strong organic acids such as the alkane, aromatic, xylene and naphthalene sulfonic acids and the trichloro-, dichloro-acetic acids, oxalic acid, fumaric acid, the Lewis acids such as aluminum chloride, zinc chloride, stannic chloride and ferric chloride. The molten mass is thereafter distilled at a pressure of from about 100 to about 760 mm. Hg and a small amount of the stabilizer [organic sulfide compound] is added to the treated and distilled product yielding a maleic anhydride composition displaying the aforesaid storage color stability." PA1 a. combining and contacting 0.05 to 2 weight percent Mg, Ca, Mn, or Fe chloride and 0.05 to 1 weight percent P.sub.2 O.sub.5 with the crude maleic anhydride to obtain a mixture, PA1 b. holding the mixture at a temperature between about 250.degree. and 350.degree.F. for between 0.1 and 12 hours, and PA1 c. distilling the mixture to obtain a purified overhead maleic anhydride fraction.
Note that in this disclosed process the maleic anhydride is first treated with a Lewis acid, and after this treatment and distillation an organic sulfide stabilizer is required for stability.
U.S. Pat. No. 3,586,703 discloses the addition of alkali and alkaline earth sulfates and halides to product commercial maleic anhydride in order to stabilize the color of maleic anhydride; U.S. Pat. No. 3,041,251 discloses heating crude maleic anhydride with sodium perborate (NaBO.sub.3) and distilling off the purified maleic anhydride; U.S. Pat. No. 3,115,477 discloses improving the color stability of maleic anhydride by: (1) treating crude maleic anhydride with from 0.1 to 10% of phosphorus pentoxide at a temperature of about 140.degree. to 200.degree.C.; (2) distilling maleic anhydride therefrom; and (3) adding color-stabilizing amounts of thiodipropionic acid or esters thereof to the distilled maleic anhydride. U.S. Pat. No. 3,622,600 discloses preparation of color-stable maleic anhydride by passing molten maleic anhydride through a bed of alkali sulfates or halides; and U.S. Pat. No. 3,636,057 discloses improving the color stability of maleic anhydride by adding trace amounts -- that is, 50 to 2,000 ppm -- of a hydrocarbyl thiophosphate to maleic anhydride.
From the above it can be seen that a very wide range of methods have been disclosed for improving the color stability of carboxylic acid anhydrides such as maleic anhydride. The use of metal halides for purification and enhancement of color stability has been disclosed, and it can also be seen by reviewing the long list that P.sub.2 O.sub.5 has been disclosed for enhancing the color stability. The use of these two specific agents together has not been disclosed nor has the use of sodium perborate with metal halides been disclosed for color stability enhancement, and U.S. Pat. No. 2,296,218 teaches that nothing is needed in addition to the use of a metal halide to obtain an essentially unchanging color -- that is, very high color stability for a product maleic anhydride.